1. Field of the Invention
The invention relates to a telecommunication network comprising a plurality of secondary stations coupled to a primary station via a channel that is shared at least partly, said secondary stations comprising a modulator for generating and supplying to the channel a carrier modulated with an auxiliary signal, the auxiliary signal comprising a digital transmit signal modulated on a subcarrier, the frequency of the subcarrier being different for different secondary stations, the primary station comprising a demodulator for deriving at least a reconstituted digital transmit signal from a signal received from the channel.
The invention likewise relates to a telecommunication station to be used in said telecommunication network.
2. Discussion of the Related Art
A telecommunication network as defined in the opening paragraph is known from the journal article "A European initiative leading to a practical customer access link using fibre" by D. E. A. Clarke, R. Mudhar and A. Purser in British Telecom Technology Journal, Vol. 11, No. 1, January 1993.
Such telecommunication systems are used for the communication between a primary station and a plurality of secondary stations via a channel used in common by the secondary stations. This channel may be formed, for example, by a glass fibre, a coaxial cable or a radio link. Applications of such communication systems are, for example, passive optical networks, local area networks, systems for satellite communication and mobile telephony systems.
In telecommunication networks which utilize a common channel for the secondary stations there is to be ensured that no or only little mutual disturbance arises as a result of secondary stations simultaneously transmitting information to the primary station.
In general, this may be effected by giving the signals from each secondary station its own frequency, so that the signals coming from the different secondary stations can be separated by frequency selective filters at the primary station. Another option is to render different time slots in a frame available which are different for each secondary station, so that only one single secondary station at a time can transmit a signal to the primary station.
A simple alternative to said methods is the telecommunication network known from said journal article, in which secondary stations are used which generate a carrier modulated on an auxiliary signal, while the carrier frequency for the different secondary stations may be about the same. This carrier is, for example, amplitude (or intensity) modulated, frequency modulated or phase modulated by an auxiliary signal in its turn obtained by the modulation of a digital signal on a subcarrier. The subcarrier is then different for the different secondary stations.
The receiver in the primary station comprises a demodulator for demodulating and separating signals coming from the different secondary stations. Separating the signals coming from the different secondary stations may be effected, for example, by bandpass filters or synchronous detectors followed by low-pass filters.
In said journal article there is mentioned that other secondary stations can be expected to cause disturbance to the signal coming from a specific secondary station. This disturbance is caused by interference from different carriers which have about the same frequency in the receiver of the primary station. The power of this interference depends, amongst other things, on the bandwidth of the auxiliary signal, the spectral bandwidth of the carrier, the carrier frequency differences of different secondary stations and on the number of secondary stations. The disturbance enhances as the number of secondary stations rises and decreases as the frequency difference between different carriers and the spectral bandwidth of the carrier become greater.
In practice the maximum transmission capacity of such a known telecommunication network is constrained by this mutual disturbance of signals coming from different secondary stations.